Electric furnace having specifically located tap hole

ABSTRACT

A method and an electric furnace for making ferro alloy, in which molten product metal and molten slag are simultaneously removed through a common tap hole bored on a wall of the furnace at a position above the level of a molten product layer in the furnace. The spacing between the tap hole and the top level of the molten product metal layer is related to the weight of the starting materials in the furnace. The molten product metal layer protects the bottom wall of the furnace against corrosion.

United States Patent [191 [11] 3,840,688 Tomioka [45] 74 {541 ELECTRICFURNACE HAVING 3,588,309 6/1971 Yasukawa..: 13/9 x SPECIFICALLY LOCATEDTAP HOLE [75] Inventor: Shigenori Tomioka, Tokyo, Japan [73] Assignee:Japan Metals & Chemicals Co.,

. v Ltd., Tokyo, Japan [22] Filed: Mar. 25, 1974 [21] Appl. No.: 454,196

Related US. Application Data [62] Division of Ser. No. 336,855, Feb. 28,1973.

[52] US. Cl. 13/9 [51] Int. Cl...... F27b 14/00 [58] Field of Search13/9, 35, 33 [56] References Cited UNITED STATES PATENTS 3,465,0859/1969 Yonemochi 13/9 X Primary Examiner-R. N. Envall, Attorney, Agent,or Firm-Ladas, Parry, Von Gehr, Goldsmith & Deschamps [57] ABSTRACT Amethod and an electric furnace for making ferro alloy, in which moltenproduct metal and molten slag are simultaneously removed through acommon tap hole bored on a wall of the furnace at a position above thelevel of a molten product layer in the furnace. The spacing between thetap hole and the top level of the molten product metal layer is relatedto the weight of the starting materials in the furnace. The moltenproduct metal layer protects the bottom wall of the furnace againstcorrosion.

6 Claims, -2 Drawing F igure's PATEN] EU OCT 81374 ELECTRIC FURNACEHAVING SPECIFICALLY LOCATED TAP HOLE This is a division of applicationSer. No. 336,855, filed Feb. 28, 1973.

BACKGROUND OF THE INVENTION 1. Field of the Invention This inventionrelates to a method of making ferro alloy by maximizing slag extractionat the time of tapping the ferro alloy from a furnace while leaving suchan amount of metallic ferro alloy in the furnace which is suitable forprotection of the furnace bottom wall. This invention also relates to anelectric furnace which is particularly suitable for carrying out theaforesaid method of making ferro alloy.

2. Description of the Prior Art Conventionally, ferro alloy is made byusing an electric furnace, e;g., an Heroult arc furnace, in which oresand carboneous reducing agents are placed. One or more slagmaking agentsare sometimes added in the furnace. The product metal is removed fromthe furnace either together with slag through a common tap hole, or theproduct metal and slag are separately removed from the furnace throughdifferent holes.

For instance, US. Pat. No. 3,167,420, that was issued to Alfred Gordonon Jan. 26, 1965, teaches a process in which product metal and slag areremoved through a common tap hole. Such a process, however, has ashortcoming in that, if entire molten metal is removed from the furnacetogether with the slag without 2 allowing operators at both the tap holeand the slagremoving hole; and that the operators must be trained foroperations at the opposite sides of the furnace. The process of theCanadian Patent No. 865,816 also has shortcoming in that the tip ofelectrodes are located at comparatively shallow positions and causelarge heat dissipation, that fresh slags which are left after removal ofmolten slag through the slag-removing hole tends to retard the reactionof forming the ferro alloy, and that the stability of the operation ispoor.

Therefore, an object of the present invention is to mitigate theaforesaid difficulties of the conventional process and furnaces ofmaking ferro alloy, by providleaving anymelt in the furnace, thevariation of thermal energy in the furnace will become excessively largeand there will be caused considerable corrosion of the furnace bottomwall. As a result, the electricpower consumptionper unit amount of themetal product increases, and the refining efficiency becomes low. Theyield of the process also becomes low. To mitigate such shortcoming, ithas'been proposed to dispose the tap hole at a position which is locatedabove the level of the molten metal product, but such modification isnot satisfactory, too. More particularly, even when the tap hole islocated at a comparatively high position, the level of the molten metalproduct varies during the refining process and uniform tapping rate ofthe molten product metal cannot be achieved. Thus, the operation of thefurnace may become instable. Furthermore, when the surface level of themolten product metal varies rapidly, the furnace bottom wall may beexposed to materials other than the molten product metal which tend tocorrode the. furnace bottom wall.

It has been proposed to provide a slag-removing hole .in a refiningfurnace which is separated from a tap hole for removing molten productmetal, as shown in Canadian Pat. No. 865,816 that was issued to JutaroYonemochi on Mar. 9, 1971. In this case, a special techniquefor sealingthe metal tap hole'is required, because the metal tap hole is requiredto bear load due to the weight of slag in the furnace. The specialsealing technique may include the use of a special sealing paste andcomplicated operation of the furnace requiring extra training ofoperators. The application of the paste to the metal tap hole needsspecial care, in order to ensure the safety of the operators. Theslagremoving hole is usually located at a diametrically oppositeposition to the metal tap position,so that various precautions must betaken: namely, that extra floor space is necessary for ing an improvedmethod and a furnace therefor.

SUMMARY OF THE INVENTOIN bored through the furnace side wall at aposition which is above the level of the top surface of the residualmolten metal pool. The vertical distan between the top sur face level ofthe residual molten metal pool and the tap hole is such that a metalextruding pressure corresponding to the weight of the starting materialsat the core portion of the furnace is applied to the tap hole.

According to the present invention, there is provided a method of makingferro alloy by using a refining electric furnace, comprising steps ofloading the furnace -with a selected amount of solid mixture consistingof starting material ore and solid reducing agents; forming a moltenmetal layer at the bottom of the furnace while forming different layerson the molten metal layer, which different layers include a molten slaglayer, a slag-soaked coke layer, a half-molten starting material layer,and a layer of mixture of solid starting materials; and removing themolten metal product together with as much slag as possible through atap hole bored through the side wall of the furnace, said tap hole beinglocated at a position-above a lowest allowable top surface level H, ofthe molten metal layer, asmeasured from the inner bottom surface of thefurnace, bya distance H which corresponds to an extruding pressureapplied to the molten metal layer by the weight of the startingmaterials for removal from the furnace, whereby the ferro alloyproduction is continuously carried out while protecting the bottomsurface of the furnace with the molten metal layer.

The present invention also relates to a refining electric furnacecomprising a vessel having side and bottom walls and an inlet openingthrough which starting materials and carboneous reducing agents'areplaced in the furnace, electrodes movably carried by a support so as tobe selectively dipped in the starting materials placed in the vessel,and a tap hole bored through the side wall of the housing at a height H(cm) from inner surface of the bottom wall of the vessel, the electrodesbeing adapted to provide electric currents through the starting materialin the furnace so as to form a mixed solid starting material layer ofmean thickness H (cm), a half-molten starting material layer of meanthickness H (cm), a molten starting material layer, a carboneous of thefurnace, said height H of the tap hole satisfying the followingrelations,

H, P,/s X 1000 here,

P an extruding pressure being applied to the molten metal,

H maximum head (cm) applicable to the molten metal layer for removalthereof from the vessel through the tap hole,

H minimum height (cm) of top level of molten product metal layer to beformed on the vessel bottom wall, as measured from the inner surface ofthe bottom wall,

Sg,: mean apparent density (g/cm of the mixed solid starting materiallayer,

Sg mean apparent density (g/cm of the halfmolten starting materiallayer,

Sg apparent density (g/cm) of the molten product metal, and

K a constant.

With the method and the furnace, according to the present invention,operation of making ferro alloy can be carried out continuously for anextended period of time, while protecting bottom wall of the furnaceagainst corrosion. It has been found that the operation of the furnaceof the present invention is very stable. At the end of the molten metalremoval, a certain amount of molten metal is left at the bottom of thevessel for providing a molten metal pool, so that the temperaturevariation between before and after the molten metal removal isminimized, as compared with the complete removal of the entire moltenmetal. The comparatively stable temperature and the removal of the slagtogether with the molten metal result in a high yield of the ferro alloyper unit refining time. With the tap hole disposed at the aforesaidposition relative to the bottom of the vessel, the head of the moltenmetal at the tap hole becomes negligible at the end of the tappingoperation so that the tape holecan easily be sealed, without causing anydanger to the operator. The distribution and balance of the differentlayers in the furnace are improved with the tap hole thus disposed,whereby the molten metal can be tapped substantially at a uniform rate.

BRIEF DESCRIPTION OF THE DRAWING For a better understanding of theinvention, reference is made to the accompanying drawing, in which:

FIG. 1 is a schematic vertical sectional view of a conventional refiningfurnace, illustrating the manner in which starting materials are refinedtherein; and

FIG. 2 is a schematic vertical sectional view of a refining electricfurnace, according to the present invention, shown in operatingconditions.

Like parts are designated by like numerals and symbols throughout thedifferent figures of the drawing.

DESCRIPTION OF THE PREFERRED EMBODIMENT When ferro alloy is made byusing an electric refining furnace, it is believed that differentstarting materials, such as semi-refined layers, molten metal layers,and molten slag layers, are distributed as shown in FIG. 1. Thedistribution of FIG. 1 is assumed, based on operating experiences andinspection of the furnace at the time of maintenance work.

More particularly, a molten metal layer 1 and a molten slag layer 2 areso formed in an electric furnace 20 that the two layers lie horizontallyat the bottom portion of the furnace, with the metal layer placed at alower position due to the difference of the density. Three furtherdifferent layers float on the molten slag layer 2; namely, a coke bed 3which is soaked with molten slag, a half-molten starting material layer4, and a solid layer of starting material mixture. It has been believedthat the solid layer 5 of the starting material mixture is graduallyheated, dehydrated, calcined, and preliminarily reduced. The lowerportion of the solid layer 5 is gradually melted to form the half-moltenlayer 4, which is then reduced when coming in contact with the coke bed3, so that it is divided into a molten slag layer 2 and a molten metallayer 1. As a tap hole 6 is opened, both the molten metal and the moltenslag are discharged through the tap hole 6, until the top surface levelof the molten metal layer 1 coincides with the level of the tap hole 6at the end of the discharge. If a large extruding or dischargingpressure should be applied to the molten metal and slag layers 1 and 2,those layers would be completely discharged, so that the inner surfaceof the bottom wall would be exposed to corrosive substances and the riskof corrosion of the bottom wall is increased.

On the other hand, the inventor has carried out a series of studies onthe dynamics of materials in the furnace during the refining operation,and he has confirmed the result of such studies by inspection of arefining furnace which had been used continuously for more than oneyear. Based on such result, the inventor has worked out a new method ofmaking ferro alloy and new dimensions for an electric refining furnace.Conventional practice of selecting different dimensions of a refiningelectric furnace is based on a theory which is developed from planarstudies of the furnace, such as studies by Morcammer and Kelley. Theconventional planar studies do not pay due attentions to more dynamicfactors of the electric furnace: for instance, the relation betweenfurnace depth and tap hole location, and the effect of molten metallayer. The inventor has carried out studies on the dynamic operation ofa refining electric furnace, and designed and prepared a new refiningelectric furnace based on the findings of such studies. The new refiningelectric furnace thus prepared proved to be able to provide molten ferroalloy at a substantially uniform rate with a high yield. The yield of aprocess using the furnace of the present invention is improved, becausethe amount of nonreduced components in the slag is reduced. Theuniformity of the delivery of the molten ferro alloy is particularlysuitable for continuous operation of such furnace.

FIG. 2 is a conceptual view of inside conditions of an electric refiningfurnace according to the present invention during a process of makingferro alloy. A furnace 20 comprises side walls 9 and a bottom wall 8. Amixed solid starting material layer 5 is formed in the furnace 20 bycharging it through an inlet opening 13. Solidified layers 7 of startingmaterials may be deposmetal layer 1 which are located vertically belowelec-.

trodes 10. The electrodes are movably carried by a support 12.Slag-soaked coke layers 3 are formed on the molten slag layers 2, so asto cover the latter. In the embodiment of FIG. 2, the central portion ofthe molten metal layer 1 carries neither one of the molten slag layer 2and the coke layer 3. Thus, the mixed layer 11 at the central portion ofthe molten metal layer 1 is covered with a half-molten starting materiallayer 4, which layer 4 also covers the molten slag layers 3. Acomparatively thick solid starting material layer 5 spreads over theentire span of the halfmolten starting material layer 4. In FIG. 2, theeffective thickness of the solid starting material layer 5 at thefurnace core and the effective thickness of the half-molten startingmaterial layer 4 at the furnace core are designated by H, (cm) and H(cm), respectively. Thus, the surface of the molten metal layer 1 isloaded with the pressure caused by the solid starting material layer 5of the thickness H, and the half-molten starting material layer 4 ofthe. thickness H irrespectively of whether it is directly beneath thefurnace core or beneath the electrodes 10. As a result, the top surfaceof the molten metal layer 1 becomes flat, and the flatness of the moltenmetal layer 1 is confirmed by calculation based on measured loads andinspection of the inner surface of the furnace at the time of itsmaintenance.

The inventor has conducted analytical studies on the dynamics oflong-term operations of different kinds of electric furnaces for makingferro alloy, which furnaces were rated from 2,500 KW to 22,000 KW. As aresult, the following conclusion was derived.

I The pressure P (Kg/c'rn which is applied to thetop surface of themolten metal layer 1 by the weight of the starting materials, is givenby the following equation here,

1 Sg,: mean apparent density of the mixed solid starting material layer5 (g/cm),

SG mean apparent density of the half-molten starting material layer 4(g/cm),

I-I,: mean thickness of the mixed solid starting material layer 5 at thefurnace core (cm), and

H mean thickness of the half-molten starting material layer 4 at thefurnace core (cm).

If the height of the top level of the molten product metal layer 1 asmeasured from the inner surface of the bottom wall 8 at the end oftapping, which height may also be referred to as a minimum height of thetop level of the molten product metal layer 1, is represented by H,(cm), a maximum head H (cm) applicable to the molten product metal layer1 for removal thereof through the tap hole 6 can be given by thefollowing equation (2). r

here, Sg represents apparent density of the molten product.

The maximum head H means that, in dynamic operation of the furnace 20, amolten product metal layer of thickness of up to (H H.,) as measuredfrom the inner surface of the bottom wall 8 can be removed through thetap hole 8.

Referring to FIG. 2, H, represents the height of the top surface of themolten product layer l from the inner surface of the bottom wall 8 atthe beginning of a tapping operation, and H represents distance betweenthe tap hole 6 and the top surface of the layer 1 at the beginning ofthe tapping operation.

The thicknesses H and H' 'of the starting material layers 5 and 4 maysomewhat vary depending on the operating conditions of the furnace andthe melting points and sizes of the starting materials, but theinspection of the furnace inside at the maintenance work provded thatsuch thicknesses are within the following ranges.

Furnace capacity Hz/(Hf z) Less than 5.000 Kw 30 35% 5,000 m 10,000 KW27 32% Above 10.000 KW 25 30% here, K is a proportionality constant.

Based on operating experiences and the inspection of the inside of thefurnace, the inventor has found out that the value of the constant K ofthe equation (3) should be 30 to 120, preferably 35 to '90. When theconstant K of 30 to is used, the amount of the prod uct metal in thelayer 1 corresponds to a product of refining operation for 6 to 24hours. When the constant K is less than 30, the bottom wall 8 of thefurnace 20 may be corroded, and the starting material layers may belowered too far and the tapping through the hole 6 may be hamperedthereby. On the other hand, if the constant K is greater than 120,solidified phases of the starting material and product metal may beformed at the central portion of the bottomwall 8 of the furnace 20.Such solidified phases of the starting materials and product metal alsotend to hamper the tapping of the molten product metal through the taphole 6. Accordingly, the constant K should be in the range of 30 to 120. I

In selecting the value of K, due care should be taken so as to providerelatively constant values of the minimore such tap holes may be boredthrough the side wall 9 of the furnace, so as to stabilize the refiningoperation in the furnace.

The invention will now be described in further detail by referring toexamples.

EXAMPLE 1 Starting materials:

Manganese ore (containing 43.8% of 100 Kg manganese) Coke Kg Lime stone6 Kg Qperating conditions:

Secondary voltage v 135 V Secondary current 60,000 to 65,000 A Duringthe operation, it was found that H 340 cm, H 140 cm, Sg 1.4 (g/cm, dry),and Sg 2.1 (g/cm). The dimensions H H and the mean densities Sg Sg wereas defined in FIG. 2 and the equations (1) and (2). Accordingly, thepressure P caused by the starting materials will be given by P (Sg, H,Sg z)/1,00O (1.4 X 340 2.1 X 140)/l,000 0.770 (Kg/cm The minimumthickness H of the molten product metal layer and the height H of thetap hole 6 were designed to be 35 cm and 160 cm, as measured from theinner surface of the bottom wall of the furnace, respectively. The meandensity of the molten product metal was 6.0 (g/cm).

After one year of continuous operation with the electric furnace, nocorrosion of the furnace bottom wall was found, and no solidifedmetalswas found at the central portion of the inner surface of thefurnace bottom wall. The molten product metal and the molten slag wereobtained substantially at constant rates. The operation proved to bestable. The electric power consumption per unit weight of the productwas 2,060 KWH/ton, and the yield of manganese was 80.5 percent, whichwere better than those with conventional furnaces.

The chemical composition of the product obtained was as follows.

Manganese Carbon Silicon Iron 74.2% 7.13% 0.35% Balance Powerconsumption ratYper tQFoFprEHuct Yield of Manganese Thus, theperformance of the furnace with the design according to thepresentinvention proved to be better than that of conventional design.Furthermore, it was found at the time of maintenance inspection that thefurnace bottom wall was corroded by 40 cm after the latter referenceoperation. The reference operation was not stable.

EXAMPLE 2 A half-sealed three-phase electric furnace of 9,500 KW ofHeroult type was prepared, in accordance with the principles of thepresent invention. Silicomanganese according to the stipulations of JIS(Japanese Industrial Standard Class 2 was prepared by using the electricfurnace, with the following starting materials and operating conditions.

Starting materials:

Manganese ore (containing 30.3% of 100 Kg manganese) Manganese slag 150Kg Silica 50 Kg Coke 43 Kg Operating conditions:

Secondary voltage 135 V Secondary current 50,000 to 52,000 A During theoperation, it was found that H 175 cm, H cm, Sg 1.35 (g/cm, dry), Sg 2.0g/cm and P, 0.386 (Kglcm The dimensions H,, I-1 and the mean densitiesSg,, Sg were as defined in FIG. 2 and the equations (1) and (2).

The minimum thickness H, of the molten product metal layer and theheight H of the tap hole 6 were designed to be 20 cm and cm, as measuredfrom the inner surface of the bottom wall of the furnace, respectively.The mean density of the molten product metal was 5.9 (g/cm").

After two years of continuous operation with the electric furnace ofExample 2, the following performance characteristics were obtained.

Electric power consumption rate Yield of manganese Composition of theproduct 3,510 KWH/ton 86.3%

The operation of the furnace of Example 2 proved to be stable, and itwas generally better than that of conventional furnaces.

For comparison, the furnace of Example 2 was operated with the tap holeheight H cm, for half a year prior to the operation of the design of thepresent invention. The performance during this half a year was asfollows: namely, the electric power consumption rate per unit weight ofthe product was 3,740 KWH/ton; the manganese yield was 80.4 percent; thefurnace bottom was raised and the excessive and insufficient outputs ofthe molten product metal were experienced in the first 2 to 3 monthsafter the start of the operation and it was stabilized only aftermodifying the at a height H (gi r minnensur al g th b ttgm wall of thevessel, the electrodes being adapted to provide electric currentsthrough the starting materials in the furnace so as to form a mixedsolid starting material layer of mean thickness H, (cm), a half-moltenstarting material layer of mean thickness H (cm), a molten startingmaterial layer, a carboneous material layer, and a molten product metallayer in the furnace in said order from the upper portion thereof, saidmean thicknesses being taken at a central portion of the furnace,

said height H of the tap hole satisfying the following conditions,

H l-l H P (Sgyl-l Sg 'H2)/1,00O H (P /Sg X 1000 H K-P here,

P extruding pressure being applied to the molten metal,

H maximum head (cm) applicable to the molten product metal layer forremoval thereof from the vessel through the tap hole,

l-l minimum height (cm) of top level of molten product metal layer to beformed on the vessel bottom wall, as measured from the inner surface ofthe bottom wall,

Sg mean apparent density (g/cm) of the mixed solid starting materiallayer,

Sg mean apparent density (g/cm) of the halfmolten starting materiallayer,

Sg apparent density (g/cm of the molten product metal, and

K a constant.

2. A refining electric furnace according to claim 1,

wherein said constant K is 30 to 120.

3. A refining electric furnace according to claim 1, wherein saidconstant K is 35 to 90.

4. A refining electric furnace according to claim 1,

wherein H /(H H is 0.30 to 0.35.

5. A refining electric furnace according to claim 1,

wherein H,j(H H is 0.27 to 0.32.

6. A refining electric furnace according to claim 1, wherein H /(H,+ His 0.25 to 0.30.

lJNITED STATESBATENT OFFICE C ERTIFICATEVOF CORRECTION Pate $840588Dated ctober-8, 1974- Invent It is certified that error appears in theabove-identified patent and that said Letters Patent are herebycorrected as shown below:

On the cover sheet, left column, below 211 A p- .;Nod 454,196"

should be added-*- -[3 0] Foreign Application Priority Data Maroh 6-,1972 Japan. ..1.... n;..'...22213 72--*.

Signed and sealed this 21st day of January 1975.

ISEAL) Attest:

MCCOY M. GIBSON; JR. Attesting Officer C. MARSHALL DANN Commissioner ofPatents STATES PATENT OFFICE. (LERTIFICATE' OF CORRECTION I D g October-8', 1974' Patent No. ,3

Inventor) Shigenori Tomioka 1' appears in the above-itlehtified patentIt is certified that erro cted as shown below:

and that said Letters Patent are hereby corre On the cover sheet, leftcolumn, below [21] App'lyNo. 454,196"

should be added-5'- --[30] Foreign Application Priority Data' Match 6,1972 Japan' ..;.".-..'...22213/72--'4 Signed and sealed this 21st day ofJanuary 1975.

ISEAL) Attest:

MCCOY M. GIBSON JR. C. MARSHALL DANN Attesting Officer Commissioner ofPatents

1. A refining electric furnace comprising a vessel having side andbottom walls and an inlet opening through which starting materials andcarboneous reducing agents are placed in the furnace, electrodes movablycarried by a support so as to be selectively dipped in the startingmaterials placed in the vessel, and a tap hole bored through the sidewall of the housing at a height H (cm) from inner surface of the bottomwall of the vessel the electrodes being adapted to provide electriccurrents through the starting materials in the furnace so as to form amixed solid starting material layer of mean thickness H1 (cm), ahalf-molten starting material layer of mean thickness H2 (cm), a moltenstarting material layer, a carboneous material layer, and a moltenproduct metal layer in the furnace in said order from the upper portionthereof, said mean thicknesses being taken at a central portion of thefurnace, said height H of the tap hole satisfying the followingconditions, H H3 + H4 P1 (Sg1.H1 + Sg2.H2)/1,000 H3 (P1/Sg3) X 1000 H4K.P1 here, P1 : extruding pressure being applied to the molten metal, H3: maximum head (cm) applicable to the molten product metal layer forremoval thereof from the vessel through the tap hole, H4 : minimumheight (cm) of top level of molten product metal layer to be formed onthe vessel bottom wall, as measured from the inner surface of the bottomwall, Sg1: mean apparent density (g/cm3) of the mixed solid startingmaterial layer, Sg2: mean apparent density (g/cm3) of the half-moltenstarting material layer, Sg3: apparent density (g/cm3) of the moltenproduct metal, and K : a constant.
 2. A refining electric furnaceaccording to claim 1, wherein said constant K is 30 to
 120. 3. Arefining electric furnace according to claim 1, wherein said constant Kis 35 to
 90. 4. A refining electric furnace according to claim 1,wherein H2/(H1 + H2) is 0.30 to 0.35.
 5. A refining electric furnaceaccording to claim 1, wherein H2/(H1 + H2) is 0.27 to 0.32.
 6. Arefining electric furnace according to claim 1, wherein H2/(H1+ H2) is0.25 to 0.30.